Sheet manufacturing method, sheet, sheet manufacturing apparatus, and solar cell

ABSTRACT

An inexpensive sheet with excellent evenness and a desired uniform thickness can be obtained by cooling a base having protrusions, dipping the surfaces of the protrusions of the cooled base into a melt material containing at least one of a metal material and a semiconductor material for crystal growth of the material on the surfaces of the protrusions. In addition, by rotating a roller having on its peripheral surface protrusions and a cooling portion for cooling said protrusions, the surfaces of the cooled protrusions can be dipped into a melt material containing at least one of a metal material and a semiconductor material for crystal growth of the material on the surfaces of the protrusions. Thus, a sheet with a desired uniform thickness can be obtained without slicing process.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a sheet manufacturing method anda manufacturing apparatus of manufacturing a sheet (a plate-like base)from a melt containing a metal material or a semiconductor material.Particularly, the present invention relates to a technique forinexpensively manufacturing solar cells, where a silicon sheet for asolar cell is directly manufactured from a silicon melt. Moreparticularly, the present invention relates to a silicon sheetmanufacturing apparatus and method enabling manufacture of a siliconsheet, which has protrusions or curved portions at least on the meltside when viewed in section.

[0003] 2. Description of the Background Art

[0004] Examples of conventional methods of manufacturing polycrystallinesilicon wafers for use in solar cells include a method of casting toform a polycrystalline body of e.g., silicon, disclosed in JapanesePatent Laying-Open No. 6-64913. In this method, a high-purity siliconmaterial containing a dopant of phosphorus, boron, or the like, isheated to melt in a crucible placed in an inert ambience. Then, asilicon melt is poured into a casting mold for gradually cooling, so asto provide a polycrystalline ingot. Accordingly, for manufacturingpolycrystalline silicon wafers for use in solar cells from thus obtainedpolycrystalline ingot, the ingot is to be sliced by a wire saw or innerdiameter blade.

[0005] A method of continuously casting a silicon plate, disclosed inJapanese Patent Laying-Open No. 7-256624, involves manufacture of asilicon sheet without slicing. In this method, a silicon melt is pouredinto a horizontal heat casting mold and a dummy graphite plate ishorizontally inserted such that its leading end is dipped into thesilicon melt under a control plate. When the silicon adheres to theleading end of the graphite plate, the silicon plate is horizontallypulled out with use of a roller. Coolant gas supplied from gas blow-offpipe of a cooling apparatus provides for continuous formation of thesilicon plate.

[0006] Another method of manufacturing a silicon sheet uses amanufacturing apparatus for a silicon ribbon disclosed in JapanesePatent Laying-Open No. 10-29895. The manufacturing apparatus for asilicon ribbon generally has a portion of heating to melt silicon and acooling roller of a heat-resistant material. The cooling roller, withone end of carbon net wound thereon, is directly dipped into a siliconmelt for forming a silicon ribbon on the surface of the cooling roller.The carbon net wound on the cooling roller is pulled with rotation ofthe cooling roller for rolling out thus formed silicon ribbon. As such,the manufacturing apparatus allows the silicon ribbon, formed from thesilicon that first adhered to the carbon net, to be continuously rolledout.

[0007] However, the above described conventional methods or apparatusesof manufacturing a silicon plate or a silicon sheet suffer from thefollowing problems. The method of casting a crystalline body of e.g.silicon disclosed in the aforementioned laid-open application No.6-64913 requires slicing of the polycrystalline ingot, whereby a slicingloss is caused corresponding to a thickness of the wire saw or innerdiameter blade. This results in yield decrease and it becomes difficultto provide wafers at low cost.

[0008] The method of continuously casting the silicon plate disclosed inthe aforementioned laid-open application No. 7-256624 controls thethickness of the silicon plate by pulling out the silicon plate underthe thickness control plate. In this case, it is difficult to control athickness of 600 μm or smaller as is employed for solar cells.

[0009] In the method of manufacturing the silicon ribbon disclosed inthe aforementioned laid-open application No. 10-29895, the siliconribbon formed from the silicon that first adhered to the carbon net iscontinuously pulled and rolled out with rotation of the cooling roller.However, the silicon ribbon is somewhat fragile due to reaction of thecarbon net and the silicon. If the formed silicon is extremely thin, thesilicon ribbon may break to fall during pulling operation. In thissituation, the operation must be stopped and productivity decreases.

[0010] Further, a mechanism is provided which pressurizes to apply thesilicon melt onto the peripheral surface of the cooling roller by jetpressure. Since the pressure is exerted by agitation of the siliconmelt, unwanted pressure may be exerted to the formed silicon, therebycausing defects.

[0011] A growth rate of silicon is determined by a number of factorsincluding a temperature of a heater for maintaining the silicon in amolten state, dipping depth, type and flow rate of coolant gascirculating in the cooling roller, rotation speed of the cooling rollerand the like. Thus, it is technically difficult to stably pull out thesilicon ribbon while controlling the growth rate.

[0012] Further, a wedge is provided for removing any silicon residueleft on the surface of the cooling roller. Such a wedge is brought intodirect contact with the surface of the cooling roller where silicongrows, thereby disadvantageously scratching the surface of the coolingroller or striping a remover applied thereon. Consequently, evenness ofthe silicon ribbon is impaired.

[0013] An inexpensive solar cell requires a base which has excellentevenness and well-controlled thickness and which saves a slicing loss.In any of these conventional cases, it is difficult to provide a thinbase with excellent evenness manufactured by mass production at lowcost.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is to overcome theaforementioned problems associated with the conventional technique, soas to provide a method of manufacturing an inexpensive sheet with adesired thickness and excellent uniformity and evenness. The methodenables manufacture of the silicon sheet with a desired thickness andexcellent evenness without slicing while ensuring productivity.

[0015] According to the present invention, the above mentioned object isachieved in the following manner.

[0016] According to one aspect of the present invention, a sheetmanufacturing method is provided in which a base with protrusions iscooled and the surfaces of the protrusions of the cooled base are dippedinto a melt material containing at least one of metal and semiconductormaterials to form crystals of the material on the surfaces of theprotrusions. Thus, a sheet of the material is produced.

[0017] According to another aspect of the present invention, a sheetmanufacturing method is provided in which a roller having on itsperipheral surface protrusions as well as a cooling system for coolingthe protrusions is rotated and the surfaces of the cooled protrusionsare dipped into a melt material containing at least one of metal andsemiconductor materials to form crystals of the material on the surfacesof the protrusions. Thus, a sheet formed of the material is produced.

[0018] According to still another aspect of the present invention, inthe sheet manufacturing method, the protrusions include at least one ofa dot-like protrusion or a linear protrusion.

[0019] According to still another aspect of the present invention, inthe sheet manufacturing method, the protrusions include at least one ofa dot-like protrusion and a linear protrusion in addition to a planarprotrusion.

[0020] According to still another aspect of the present invention, inthe sheet manufacturing method, the protrusions are coated with amaterial of at least one of silicon carbide, boron nitride, siliconnitride, and pyrolitic carbon.

[0021] According to still another aspect of the present invention, inthe sheet manufacturing method, the crystals of the material grow fromthe protrusions.

[0022] According to still another aspect of the present invention, asheet is provided which has curved portions formed by cooling a basewith protrusions, dipping the surfaces of the protrusions of the cooledbase into a melt material containing at least one of metal andsemiconductor materials, and forming crystals of the material in acurved shape on the surface of the base from the protrusions.

[0023] According to still another aspect of the present invention, asheet is provided which has curved portions and planar portions formedby cooling a base with protrusions including at least one of a dot-likeprotrusion and a linear protrusion in addition to a planar protrusion,dipping the surfaces of the protrusions of the cooled base into a meltmaterial containing at least one of metal and semiconductor materials,forming crystals of the material on the surface of the base in a curvedshape from the dot-like protrusion or linear protrusion, and formingcrystals of the material on the surface of the base in a planar shapefrom the planar protrusion.

[0024] According to still another aspect of the present invention, asheet manufacturing apparatus is provided which includes: a rollerhaving on its peripheral surface protrusions and a cooling system forcooling the protrusions; and a crucible into which includes a materialcontaining at least one of metal and semiconductor material and whichallows the protrusions to be dipped into the melt by rotation of theroller.

[0025] According to still another aspect of the present invention, asolar cell is provided by forming an electrode on a sheet having acurved portion formed by cooling a base with protrusions, dipping thesurfaces of the protrusions of the cooled base into a materialcontaining at least one of a metal material and a semiconductormaterial, and forming crystals of the material on the surface of thebase in a curved shape from the protrusion.

[0026] According to still another aspect of the present invention, asolar cell is provided by forming an electrode on a planar portion of asheet with a curved portion and a planar portion obtained by cooling abase with protrusions including at least one of dot-like protrusions andlinear protrusions in addition to planar protrusions, dipping thesurfaces of the protrusions of the cooled base into the melt materialcontaining at least one of a metal material and a semiconductormaterial, forming crystals of the material on the surface of the base ina curved shape from the dot-like protrusion or the linear protrusion,and forming crystals of the material on the surface of the base in aplanar shape from the planar protrusion.

[0027] According to still another aspect of the present invention, asilicon sheet manufacturing apparatus is provided for manufacturing asilicon sheet by rotating a cooling roller to solidify and form asilicon sheet. The apparatus is characterized in that the surface of thecooling roller is provided with protrusions arranged in a dot-likepattern or a linear pattern when viewed from above.

[0028] According to still another aspect of the present invention, thesilicon sheet manufacturing apparatus is characterized in that spacesbetween the protrusions are in a V or U like shape (such spaces arehereinafter referred to as V or U grooves).

[0029] According to still another aspect of the present invention, thesilicon sheet manufacturing apparatus is characterized in that thesurface of the cooling roller is coated with SiC.

[0030] According to still another aspect of the present invention, thesilicon sheet manufacturing apparatus is characterized in that the pitchof the V or U groove is at least 0.05 mm and at most 5 mm.

[0031] According to still another aspect of the present invention, thesilicon sheet manufacturing apparatus is characterized in that theheight of the protrusions is at least 0.05 mm and at most 5 mm.

[0032] According to still another aspect of the present invention, asilicon sheet manufacturing method is provided for manufacturing asilicon sheet by rotating a cooling roller to solidify and form crystalsof a silicon melt. The method is characterized in that the crystals aresolidified and formed from protrusions of the cooling roller arranged ina dot-like pattern or a linear pattern when viewed from above.

[0033] According to still another aspect of the present invention, asolar cell is provided by forming an electrode on a silicon sheetobtained by rotating a roller having on its peripheral surfaceprotrusions including at least one of a dot-like protrusion and a linearprotrusion as well as a cooling portion for cooling the protrusions,dipping the surfaces of the cooled protrusions, and forming siliconcrystals on the surfaces of the protrusions.

[0034] For producing a silicon sheet, the surface of the cooling rollerfor forming the silicon sheet has protrusions arranged in a dot-likepattern or a linear pattern and a silicon melt is solidified at theprotrusions, so that curved protrusions are formed at least on one sideof the silicon sheet. The silicon sheet of the present invention ischaracterized by a curved surface formed on the side of the siliconmelt. The continuous protrusions serve as ribs to save the siliconmaterial used and to enable mass production of silicon sheets withreduced thickness and yet sufficient strength.

[0035] The silicon sheet manufacturing apparatus according to thepresent invention may include: a roller having on its peripheral surfaceprotrusions including at least one of a dot-like protrusion and a linearprotrusion as well as a cooling portion for cooling the protrusions; anda crucible containing a silicon melt and capable of dipping theprotrusions into the silicon melt by rotation of the roller.

[0036] The cooling roller is characterized in that the spaces betweenthe protrusions on the surface of the cooling roller dipped into thesilicon melt are V or U grooves. The protrusions with V or U grooves areformed in the surface of the cooling roller in a dot-like pattern or ina linear pattern with respect to a rotation direction. The top portionsof the protrusions are dipped into the silicon melt, which are in turncooled from inside. This produces crystal nuclei of silicon at the topportions of the protrusions, which gradually grow and combine together,i.e., one nuclei combines with those growing from the adjacent topportions. Thus, a silicon sheet is produced. In this way, stable growthof the silicon sheet is enabled.

[0037] Further, use of the cooling roller having the V or Ugroove-structure enables manufacture of a corrugation or wave-likesilicon sheet directly on the cooling roller, depending on manufacturingconditions of the silicon sheet. Accordingly, the sheet has curvedportions not only on the melt side but also on the side of the coolingroller, in accordance with the above mentioned dot-like pattern orlinear pattern. This is due to changes in shape of the silicon crystalsgrowing from peaks of the V or U grooves. Such curved portions providefor significant reduction in silicon material used.

[0038] In addition, the following method further facilitatesindustrialization. A silicon carbide (SiC) coating is applied to thesurface of the cooling roller. According to the above mentionedstructure, the presence of the SiC film on the surface of the coolingroller prevents contamination by the material used for the coolingroller. The pitch of the V or U grooves of the cooling roller is atleast 0.05 mm and at most 5 mm. According to the structure, the crystalnuclei of the silicon are produced only at the peaks of the V or Ugrooves of the cooling roller, so that the grain size or thickness ofthe silicon sheet can readily be controlled.

[0039] In the structure, although a pitch less than 0.05 mm may increasethe number of protrusions to be dipped into the silicon melt to achievehigher growth rate, it is not desirable since the grain size wouldbecome extremely small. Although a pitch greater than 5 mm may provide agreater grain size, in this case, a problem associated with the growthrate arises.

[0040] In the structure, the height of the protrusions of the coolingroller, i.e., the depth of the V or U grooves is desirably at least 0.05mm and at most 5 mm.

[0041] In the structure, only the peaks of V or U grooves of the coolingroller can be reliably dipped into the silicon melt in a portion forheating and melting silicon. The depth smaller than 0.05 mm maydisadvantageously cause troughs of the V or U grooves of the coolingroller to be dipped into the silicon melt. As a result, a contact areaincreases and removability is impaired.

[0042] As in the foregoing, by controlling the surface structure of thecooling roller and properly setting a dipping condition or the like, asilicon sheet with a moderate thickness is produced.

[0043] The silicon sheet manufacturing apparatus further includes abar-like remover for a silicon sheet, which is disposed at the trough ofthe cooling roller.

[0044] In the structure, the crystal nuclei of silicon are produced atthe peaks of V or U grooves of the cooling roller, which gradually growto combine with those from adjacent peaks to form a silicon sheet. Thusformed silicon sheet is readily removed as a single sheet from the V orU grooves of the cooling roller.

[0045] In this case, the remover is disposed at the trough of thecooling roller. Accordingly, the remover would not cause any damage tothe peaks of the V or U grooves at which crystals grow. In addition, thepresence of the SiC coating on the cooling roller can prevent asubstance at the trough from falling.

[0046] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a perspective view showing a base with linearprotrusions of the present invention.

[0048]FIG. 2 is a perspective view showing a base with dot-likeprotrusions of the present invention.

[0049]FIG. 3 is a perspective view showing a base with linearprotrusions and planar protrusions of the present invention.

[0050]FIG. 4 is a perspective view showing a base with dot-likeprotrusions and planar protrusions of the present invention.

[0051]FIG. 5A is a schematic perspective view showing a sheet, and FIG.5B is a schematic perspective view showing a base with linearprotrusions of the present invention.

[0052]FIG. 6A is a schematic perspective view showing a sheet, and FIG.6B is a schematic perspective view showing a base with linearprotrusions of the present invention.

[0053]FIG. 7A is a schematic perspective view showing a sheet, and FIG.7B is a schematic perspective view showing a base with linearprotrusions of the present invention.

[0054]FIG. 8A is a schematic perspective view showing a sheet, and FIG.8B is a schematic perspective view showing a base with dot-likeprotrusions of the present invention.

[0055]FIG. 9A is a schematic perspective view showing a sheet, and FIG.9B is a schematic perspective view showing a base with dot-likeprotrusions of the present invention.

[0056]FIG. 10A is a schematic perspective view showing a sheet, and FIG.10B is a schematic perspective view showing a base with dot-likeprotrusions of the present invention.

[0057]FIG. 11A is a schematic perspective view showing a sheet, and FIG.11B is a schematic perspective view showing a base with linear andplanar protrusions of the present invention.

[0058]FIG. 12A is a schematic perspective view showing a sheet, and FIG.12B is a schematic perspective view showing a base with dot-like andplanar protrusions of the present invention.

[0059]FIGS. 13 and 14 are schematic perspective views each showing asolar cell with an electrode arranged on the sheet of the presentinvention.

[0060] FIGS. 15 to 17 are perspective views each showing a base withlinear and planar protrusions of the present invention.

[0061]FIG. 18A is a schematic perspective view showing a sheet, and FIG.18B is a schematic perspective view showing a base with linear andplanar protrusions of the present invention.

[0062]FIG. 19A is a schematic perspective view showing a sheet, and FIG.19B is a schematic perspective view showing a base with dot-like andplanar protrusions of the present invention.

[0063]FIG. 20A is a schematic perspective view showing a sheet, and FIG.20B is a schematic perspective view showing a base with linear andplanar protrusions of the present invention.

[0064]FIG. 21A is a schematic perspective view showing a sheet, and FIG.21B is a schematic perspective view showing a base with linear andplanar protrusions of the present invention.

[0065]FIG. 22A is a schematic perspective view showing a sheet, and FIG.22B is a schematic perspective view showing a base with linear andplanar protrusions of the present invention.

[0066]FIG. 23A is a schematic perspective view showing a sheet, and FIG.23B schematic perspective view showing a base with linear and planarprotrusions of the present invention.

[0067]FIG. 24 is a schematic perspective view showing a sheetmanufacturing apparatus which employs a horizontal pulling method of thepresent invention.

[0068]FIG. 25 is a schematic perspective view showing a sheetmanufacturing apparatus which employs a rotation method of the presentinvention.

[0069]FIG. 26 is a cross sectional view showing a sheet manufacturingapparatus provided with a cooling portion of the present invention.

[0070] FIGS. 27 to 29 are schematic perspective views each showing asheet manufacturing apparatus provided with a cooling portion of thepresent invention.

[0071]FIG. 30 is a cross sectional view showing a general structure ofthe sheet manufacturing apparatus of the present invention.

[0072]FIG. 31 is a cross sectional view showing the sheet manufacturingapparatus provided with a cooling portion of the present invention.

[0073]FIGS. 32 and 33 are cross sectional views each showing a completedsilicon sheet and V grooves of the cooling roller of the presentinvention.

[0074]FIG. 34 is a schematic cross sectional view showing a siliconsheet manufacturing apparatus of the present invention.

[0075]FIG. 35 is a chart showing a manufacturing process of a solar cellaccording to the present invention.

[0076]FIG. 36 is a perspective view showing a base with linear andplanar protrusions of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0077] The embodiments of the present invention will now be described indetail. The present invention is a sheet manufacturing method ofdirectly forming on the surface of a cooled base crystals of a meltmaterial containing at least one of a metal material and a semiconductormaterial and is characterized by the surface structure of the base. Morespecifically, the surface of the base has protrusions which are arrangedin a dot-like pattern or a linear pattern when viewed from above.

[0078] The linear protrusions refer to protrusions formed by grooves ina planar surface of the base. The concept of the linear protrusionsinclude polishing and cutting formed on the surface of the base bymachine processing of polishing or cutting. Namely, the linearprotrusions include every protrusion in the surface of the base. Thelinear protrusions or dot-like protrusions are formed in the followingmanner. For example, when grooves having a width of 1 mm are machined inthe planar surface of the base, the linear protrusions having a pitch of1 mm are formed in the surface of the base. Further, the base with thelinear protrusions is rotated for example by 90° and the grooves havinga width of 1 mm are similarly formed. As a result, dot-like protrusionsare formed having a pitch of 1 mm. The grooves in the surface of theplanar base provide the linear or dot-like protrusions. Further, suchlinear or dot-like protrusions may be polishing traces or cutting tracescaused by polishing or cutting. Namely, the concept of the protrusionsas used herein includes any protrusion in the surface of the base.

[0079] Referring to FIGS. 1 and 2, a feature of the base of the presentinvention will be described. The bases shown in FIGS. 1 and 2 haveprotrusions in their respective surfaces. The base shown in FIG. 1 haslinear protrusions, whereas the base shown in FIG. 2 has dot-likeprotrusions.

[0080] In FIGS. 1 and 2, reference numerals 1, 2, 3, and 4 respectivelydenote the base with protrusions, linear protrusions of the base,grooves of the base, and dot-like protrusions of the base. Note thatpeaks of linear protrusions 2 and dot-like protrusions 4 may be rounded.

[0081] In the case of a planar base without any protrusions formed inthe surface of base 1, a sheet is formed by crystal growth on the planarbase. In such a case, however, it would be difficult to remove the sheetfrom the base due to relatively high adhesion strength of the sheet andthe base. In addition, in the case of crystal growth on the planar base,crystal nuclei may be randomly produced on the base. Consequently,dendritic growth becomes dominant, resulting in large protrusions of thesheet and making it difficult to form crystals around those formed bydendritic growth. Thus, it is difficult to produce, with use of theplanar base, a sheet having uniform protrusions which can be easilyremoved from the base.

[0082] At least one of linear protrusion 2 and dot-like protrusion 4facilitates removal of the sheet from the base. This is because at leastone group of linear protrusion 2 and dot-like protrusion 4 enablessetting/control of the starting point of crystallization (i.e., a pointat which crystal nuclei is produced). Further, at least one group oflinear protrusions 2 and dot-like protrusions 4 promotes nucleiproduction at the peak of the protrusions since such protrusion is firstdipped into the melt. The protrusions provide improved removability ofthe sheet from the base and facilitate control of uniformity of theprotrusions.

[0083] Although the protrusions in the surface of the base shown inFIGS. 1 and 2 form V grooves, the present invention is not particularlylimited to such configuration. The depths of the grooves in the base arethe same, while not limiting. The depths of the grooves may vary asnecessary according to the material used.

[0084] Now, the feature of the base of the present invention will bedescribed with reference to FIGS. 3 and 4. The bases shown in FIGS. 3and 4 have protrusions in their surfaces. A base 7 of FIG. 3 has linearprotrusions 2 and planar protrusions 6. The base of FIG. 4 has dot-likeprotrusions 4 and planar protrusions 6. Planar protrusions 6 refer toprotrusions formed in the surface of the base as a result of formationof grooves.

[0085] In FIGS. 3 and 4, reference numeral 6 denotes the planarprotrusions. The protrusions have at least one of dot-like protrusionsand linear protrusions, and must have planar protrusions. In the presentinvention, at least one of linear protrusions 2 and dot-like protrusions4 are arranged in addition to planar protrusions 6.

[0086] At least one group of linear protrusions 2 and dot-likeprotrusions 4 facilitates removal of the sheet from the base. Planarprotrusions 6 further facilitate removability control.

[0087] Next, referring to FIG. 5, a sheet produced using base 1 withprotrusions shown in FIG. 1 will be described. FIG. 5 is a perspectiveview showing a positional relationship between base 1 with linearprotrusions 2 and a sheet 9 produced by crystal growth on the surface ofbase 1.

[0088] A sheet produced by solidification of the melt and crystal growthon base 1 has curved portions. This is because crystals grow from linearprotrusions 2 of base 1 in a curved shape. Thus, the produced sheet hascurved portion.

[0089] Crystal growth on the base occurs in the following manner.Namely, crystal nuclei are produced at peaks of linear protrusions 2since these protrusions are first dipped into the melt. Then, since base1 is cooled, crystal growth occurs from the crystal nuclei. The curvedcrystals on linear protrusions 2 grow to combine with those fromadjacent linear protrusions 2, so as to produce sheet 9 with curvedportions.

[0090] The shape of sheet 9 does not conform to the protrusions of base1 as shown in FIG. 5. The shape of sheet 9 is approximately as shown inFIG. 5, depending on various factors including surface tension of themelt, the temperature of the cooled base, the moving or rotating speedof the base, the shape of the protrusions and the like.

[0091] This is because crystal growth starts at the peaks of linearprotrusions 2 and the melt solidifies before crystal growth proceedsdeeper in the grooves of the protrusions, and thus sheet 9 is produced.When sheet 9 is produced by use of base 1 with linear protrusions 2, acontact area between the base and the sheet would be smaller than in thecase of the planar base. Smaller contact area of the base and the sheetfacilitates removal of the sheet from the base.

[0092] If the temperature of the cooled base or the melt is high, or ifthe moving or rotating speed of the base is low, the melt willdisadvantageously proceed deeper in the grooves. In this case,removability of the sheet from the base would be impaired.

[0093] On the contrary, if the base is sufficiently cooled down, themoving or rotating speed of the base is high, and the temperature of themelt is low, crystal growth rapidly proceeds after formation of crystalnuclei to produce a sheet. However, if sufficient time is not allowedbefore crystal growth into a sheet such as when the moving speed of thebase is too high, crystal growth over linear protrusions 2 isinsufficient as shown in FIG. 6. As a result, bar-like crystals areproduced. In some cases, crystals would not grow partially over thegrooves as shown in FIG. 7.

[0094] On the other hand, even if the cooling of the base isinsufficient, a sheet can be still produced by lowering the moving speedof the base or the rotating speed of the roller having the coolingsystem, or further decreasing the melt temperature. In the above, thepresent invention has been described as using the base of FIG. 1 toproduce sheet 9 with reference to FIG. 5. In the case of the base ofFIG. 2, crystal nuclei are first formed at dot-like protrusions 4,rather than linear protrusions 2, as shown in FIG. 8. However,subsequent crystal growth occurs as in the case of the base of FIG. 1.

[0095] Namely, crystal nuclei is first formed on cooled dot-likeprotrusion 4 and then crystal growth occurs based on the crystal nuclei.As crystal growth proceeds, crystals formed from dot-like protrusions 4combine to form a sheet 12. Thus produced sheet 12 has curved portions.

[0096] Higher degree of removability of the sheet from the base isattained in the case of dot-like protrusions 4 than in the case oflinear protrusions 2. The removability largely depends on the contactarea between the sheet and the base.

[0097] If the substrate is sufficiently cooled, the moving or rotatingspeed of the base is high, and the melt temperature is low, rapidcrystal growth occurs after formation of crystal nuclei to produce asheet. However, if sufficient time is not allowed before crystals growinto a sheet as when the moving speed of the base is too high, crystalsgrow only over the dot-like protrusions 4 as shown in FIG. 9 to producecrystals roughly in the shape of balls, or the produced sheet maypartially have such ball-like portions as shown in FIG. 10, resulting indiscontinuous crystal sheet.

[0098] Now, referring to FIG. 11, a sheet produced with use of base 7 ofFIG. 3 will be described. FIG. 11 is a perspective view showing apositional relationship between the base with planar and linearprotrusions and a sheet produced by crystal growth on the surface of thebase.

[0099] A sheet 15 produced by solidification and crystal growth of themelt on the surface of base 7 has both curved and planar portions. Thisis because the crystals formed from the planar protrusions of the baseproduce a planar portion but those formed from the linear protrusionsproduce a curved portion. Thus, a single sheet has both planar andcurved portions.

[0100] Crystal growth of the sheet on the base occurs as follows.Namely, crystal nuclei first produced on the planar and linearprotrusions which are first dipped into the melt. Since the base iscooled down, crystal growth starts based on the crystal nuclei. Thecrystals in the planar shape from the planar protrusions and those inthe curved shape from the linear protrusions combine together as theygrow to produce a sheet with both planar and curved portions.

[0101] However, resultant sheet 15 would not have the shape thatconforms to the protrusions as shown in FIG. 11. Sheet 15 would have ashape generally as shown in FIG. 11, depending on various factorsincluding surface tension of the melt, temperature of the cooled base,moving or rotating speed of the base, the shape of the protrusions andthe like.

[0102] In the above, the formation of the sheet with use of base 7 ofFIG. 3 has been described with reference to FIG. 11. When a sheet isproduced with use of base 8 of FIG. 4, crystal nuclei are first formedon the dot-like protrusions, rather than linear protrusions, as shown inFIG. 12, subsequent crystal growth is similar to the case of the base ofFIG. 3.

[0103] More specifically, crystal nuclei are first formed on the cooledplanar protrusions 6 and dot-like protrusions 4, and crystal growthstarts based on the crystal nuclei. As crystal growth proceeds, crystalsfrom adjacent planar protrusions 6 and dot-like protrusions 4 combineone another to form a sheet. Note that the crystals formed from thelinear or dot-like protrusions are curved in shape and tend to havesmall grain size. Thus, it is preferred that the planar protrusionsoccupy a greater percentage of the base area.

[0104] In the case of bases 7 and 8 respectively illustrated in FIGS. 3and 4, the crystals formed from the planar protrusions produce a planarportion, whereas those from the dot-like protrusions or linearprotrusions produce a curved portion. Thus, a completed sheet has bothcurved and planar portions. Such a sheet with planar and curved portionshas enhanced strength because of the planar portion.

[0105] Next, a method of forming an electrode onto the resultant sheetwill be described with reference to FIGS. 13 and 14. FIG. 13 is aperspective view of sheet 9 with electrode 17 formed thereon. FIG. 14 isa perspective view of a sheet with a planar portion on which anelectrode 18 is formed.

[0106]FIG. 13 shows four electrodes, which are parallel to the recessesof the sheet, along with one additional electrode arranged orthogonal tothese electrodes. However, the method of forming the electrode is notparticularly limited to this. Namely, electrodes may be formed in therecesses of the sheet.

[0107]FIG. 14 shows electrode 18 formed on the planar portion of thesheet. Such arrangement prevents breakage of electrode 18 and, inaddition, electrode 18 on sheet 15 is not large in width.

[0108] The planar protrusions of the base produce planar portions of thesheet, so that the electrode can be readily formed thereon. The methodof forming the electrode may employ for example screen printing, vapordeposition or plating, while not limiting.

[0109] Reinforcing ribs of the sheet of the present invention permitusage of a method of forming an electrode with a screen mask being incontact, such as screen printing, where reduction of cost may beachieved.

[0110] When a solar cell is manufactured with use of the produced sheet,preferably, an electrode on the side of incident light (hereinafterreferred to at a front electrode) is narrow, whereas a back electrode onthe opposite side is formed over the entire area of the sheet. Bothsides of the sheet with protrusions are similarly shaped and henceeither side may be used as incident light side. However, the base sideis preferably used as the back electrode side of the solar cell. This isbecause crystal nuclei are produced on linear, dot-like, and planarprotrusions, resulting in extremely small crystal grains. Thus, a sheetmanufactured by the sheet manufacturing method of the present inventionis extremely effective for an electronic device which requires anelectrode because the electrode can be formed directly on the planarportion.

[0111] Sheet 15 of FIG. 14 is useful in that the base for manufacturinga solar cell may have a planar portion, which will be the light incidentside to have a grid-like electrode thereon. Although such a base may bemanufactured from a single crystal sheet or polycrystalline sheet, suchmanufacture disadvantageously involves use of an oxide film, patterningby application of resist or the like, and further removal of anyunwanted portion by etching. In addition, the sheet produced by thepresent invention is polycrystalline and hence includes crystal grainsof various plane directions. Such variation in plane directions resultsin variation in etching speed during patterning, and therefore it isdifficult to produce a sheet by the steps shown in conjunction withFIGS. 5 to 14. In the sheet manufacturing method of the presentinvention, an inexpensive sheet in a shape as shown in FIGS. 5 to 14 isproduced from the melt.

[0112] Here, detailed description of the protrusions of the base isgiven. Referring to FIG. 1, a space between adjacent linear protrusions2 of base 1 is not particularly limited. However, a constant pitch oflinear protrusions 2 provides better uniformity of the sheet and istherefore more preferable. Even if the pitches of linear protrusions 2are irregular, uniformity and evenness may be ensured by formation ofthe planar protrusions as shown in FIG. 3 or a relatively large width(area) of the planar protrusion. By appropriately determining the pitchof the linear protrusions, presence/absence of the planar protrusions,and the width (area) of the planar protrusions, uniformity is controlledand the sheet in a desired shape can be obtained. Similarly, in the caseof FIGS. 2 and 4, by appropriately determining the pitch of the linearprotrusions, presence/absence of the planar protrusions, and the width(area) of the planar protrusions, uniformity is controlled and the sheetin a desired shape can be obtained.

[0113] The space between the adjacent protrusions significantly governsremovability of the base from the sheet. Namely, if the pitch of thelinear protrusions is too small, the sheet is in contact with the baseover a greater area, whereby removal of the sheet from the base requiresgreater force. On the other hand, if the pitch of the adjacent linearprotrusions is too large, a relatively long time is required to allowcrystal to grow on the base. To reduce the time required for crystalgrowth, the base temperature, melt temperature or the moving speed ofthe base must be decreased. As a result, productivity decreases. Notethat the pitch of the adjacent dot-like protrusions significantlygoverns removability of the sheet from the base.

[0114] Although the pitch of the adjacent planar protrusions 6 as shownin FIGS. 3 and 4 is not particularly limited, greater width (area) ofplanar protrusions 6 provides better evenness of the sheet. On the otherhand, relatively large width (area) of planar protrusions 6 tends todecrease removability of the sheet from the base, and therefore thewidth (area) of planar protrusions 6 must be properly selected. Althoughbases of FIGS. 3 and 4 have linear or dot-like protrusions between theadjacent planar protrusions 6, the numbers thereof are not particularlylimited, and the protrusions can be designed as appropriate consideringevenness, removability or the like.

[0115] When a solar cell is to be manufactured from the resultant sheet,the pitch of the linear protrusions is preferably at least 0.05 mm. Thepitch of the dot-like protrusions is also preferably at least 0.05 mm. Apitch less than 0.05 mm results in disadvantageously small crystalgrains that impair the properties of the solar cell, and is notdesirable.

[0116] Although the recesses shown in FIGS. 1 to 4 are in the shape of Vlike grooves, the recesses may be in the shape of V as shown in FIGS. 15to 17, U, trapezoid, oblique groove and the like, and are notparticularly limited. Bases having linear and linear protrusions with Ugrooves, trapezoidal grooves and oblique grooves are respectivelydenoted by reference numerals 41, 42 and 43. The depth of the groove isat least 0.05 mm and, more preferably, at least 0.1 mm. A depth of thegroove less than 0.05 mm would make it difficult to remove the producedsheet from the base due to crystal growth at the groove. However, thedepth of the groove must be appropriately selected in consideration ofthe structure of the base and the melt material since surface tension ofthe melt material may also determine the resulting effect. Although thegrooves are shown in the drawing as having the same depth, they may havedifferent depths. This is because the temperature more or less variesover the surface of the base according to the depth of the groove whenthe base is cooled. Thus, the surface shape and the depth of the groovemust be appropriately determined for temperature control.

[0117] Further, although planar protrusions 6 are arranged orthogonallyin FIGS. 3 and 4, they may not be orthogonal. FIGS. 18 to 23 showschematic illustrations of a sheet obtained when the planar protrusionsare not orthogonally arranged. Bases with protrusions are denoted by 44,46, 48, 50, 52, and 54, whereas sheets resulting from these bases aredenoted by 45, 47, 49, 51, 53 and 55.

[0118] The planar protrusions may be arranged in parallel as shown inFIGS. 18 and 19. They may be arranged in parallel with the grooves asshown in FIG. 20. They may be formed orthogonally to the grooves asshown in FIGS. 21 and 22. Grooves may be partially formed as shown inFIG. 23. When a solar cell is to be produced from the sheet having alarge area, it is preferred that the planar protrusions are orthogonallyarranged. This is because the maximum possible current must be derivedto improve the properties of the solar cell. More specifically,orthogonal arrangement of a bus bar electrode (main electrode) and afinger electrode (sub electrode) facilitates designing to reduce seriesresistance. Thus, it is preferred that the width of the planarprotrusion is designed to reduce the series resistance. Namely,preferably, the portion to be the bus bar electrode has a larger width,whereas the portion to be the finger electrode has a smaller width.Thus, a solar cell with improved properties can be obtained byappropriately determining the width of the planar protrusion accordingto the purpose.

[0119] As stated above, the protrusions of the base act as basis forcrystal growth of the sheet, and crystal growth depends on the shape,width and pitch of the protrusions, the melt material, the melttemperature, base material, base temperature and the like. Therefore, itis preferable that these factors are appropriately determined.

[0120] By preliminary determining the shapes of the protrusions of thebase, distribution of crystal nuclei as a basis for crystal growth canbe controlled, whereby a sheet with excellent evenness and removabilityis produced.

[0121] Preferably, a material with excellent heat conductivity and heatresistance is used for the base with protrusions. Examples of materialsused for the base include high-purity graphite, silicon carbide, quartz,boron nitride, alumina, zirconium oxide, and metal. The optimum materialmay be selected according to the melt used.

[0122] High-purity graphite is more preferable because of itsinexpensiveness and workability, which facilitate formation ofprotrusions. The material for the base can be appropriately selected byconsidering industrial inexpensiveness, various properties such as abase property of the produced sheet and the like, for determining thecombination of the melt material and the base. Metal may be used for thebase without causing any problem, as long as constant cooling ismaintained, temperature of the metal is maintained below a melting pointof the base, and it does not adversely affect the properties of thesheet.

[0123] Generally, the base can be cooled either by direct or indirectcooling. For direct cooling, a base is directly cooled by gas. Forindirect cooling, the base is indirectly cooled by gas or melt. Althoughthe type of the coolant gas is not particularly limited, for the purposeof preventing oxidation of the sheet, inert gas such as nitrogen, argon,or helium is preferred. Helium or mixture gas of helium and nitrogen ispreferred particularly in terms of cooling ability, but nitrogen ispreferable in terms of cost. The coolant gas is circulated with use of aheat exchanger or the like, for cost reduction.

[0124] Now, a sheet manufacturing apparatus using a base withprotrusions will be described. Referring to a schematic diagram of FIG.24 showing a sheet manufacturing apparatus for horizontal pullingmethod, a method of forming a sheet onto a continuous base havingcontinuously arranged bases will be described.

[0125] Referring to FIG. 24, a continuous base having protrusions isdenoted by 19, a crucible by 20, and a melt by 21. The melt in thecrucible is heated to at least a melting point by a heater andmaintained in a stable melt state. Examples of the melt includesemiconductor materials such as silicon, germanium, gallium, arsenic,indium, phosphorus, boron, antimony, zinc, tin, or a melt includingmetal material such as aluminum, nickel, or iron.

[0126] The bottom of crucible 20 has an opening, through which melt 21can be continuously supplied onto continuous base 19. Crystals grow oncontinuous base 19 to form a sheet. By controlling the moving speed ofthe crucible, temperature of the continuous base, the melt temperatureor the like, the thickness of the sheet can readily be controlled.

[0127] A method of forming a sheet onto a base when a cooling rollermethod is employed will be described with reference to FIG. 25. FIG. 25is a schematic diagram showing a sheet manufacturing apparatus providedwith a movable roller. A movable roller is denoted by 22, andprotrusions in the surface thereof are denoted by 23. The protrusionsinclude both linear and planar protrusions here. Although the roller isshown as having protrusions in its surface, these protrusions may beremovably attached to the surface of the roller. Protrusions extendoutwardly with respect to a rotation axis of the roller on theperipheral surface of the roller.

[0128] An internal structure of the roller in FIG. 25 will be describedwith reference to FIG. 26. FIG. 26 is a cross sectional view showing acooling portion of the sheet manufacturing apparatus with a rollerhaving the cooling system. Coolant gas nozzle is denoted by 24. Coolantgas nozzle 24 serves to gradually cool from inside roller 22, forindirectly cooling protrusions in the surface of roller 22. Suchindirect cooling of protrusions enables continuous production of thesheet. A cooling means inside roller 22 may be any means so long as ituniformly cools the base mounted on the roller. Namely, a structure ofcooling by gas or melt is preferred.

[0129] Preferably, inert gas such as nitrogen, argon, or helium is usedas coolant gas. While helium or mixture gas of helium and nitrogen ispreferred in terms of cooling ability, nitrogen is more preferable interms of cost. Further, coolant gas or water is circulated with use of aheat exchanger or the like for cost reduction. Although the surfacestructure having linear and planar protrusions has been described withreference to FIGS. 25 and 26, only linear protrusions may be formed asshown in FIG. 31. Further, the protrusions of the base may be as shownin any of FIGS. 1 to 4 and FIGS. 15 to 23.

[0130] In the above, the structure of the surface of the roller has beendescribed. Any structure may be employed so long as protrusions areformed on the side for crystal growth of the sheet. As shown in FIGS. 26and 31, protrusions need only be formed in the surface of the roller.FIG. 26 shows linear and planar protrusions formed in the surface,whereas FIG. 31 shows only linear protrusions formed in the surface.While not shown in the drawings, dot-like protrusions may be formed.

[0131] The cooling roller, being cylindrical in shape, has beendescribed in the above. Now, polygonal cooling roller will be described.In this case, since the side for crystal growth is planar, a planarsheet is obtained. Thus, a polygonal roller is extremely effective forthe purpose of manufacturing solar cells.

[0132]FIG. 27 is a schematic diagram showing a sheet manufacturingapparatus provided with a movable polygonal roller. The roller withlinear protrusions is denoted by 56. Although the roller is shown ashaving four sides adapted to be used for crystal growth of the sheet,the roller may not necessarily have four sides. In terms ofproductivity, a roller with a large diameter is preferably employed sothat one rotation provides a plurality of sheets. FIG. 27 shows linearprotrusions formed on the roller having planar surfaces. In this case,the linear protrusions are arranged in the rotation direction of theroller.

[0133]FIG. 28 shows linear protrusions formed orthogonal with respect tothe rotation direction of the roller. A roller with linear protrusionsis denoted by 57. When a sheet is to be formed on the linearprotrusions, such linear protrusions are preferably formed in the samedirection as the rotation direction of the roller. If the linearprotrusions are in the rotation direction, the melt more easily spreadover the surface of the base with protrusions. In other words, the meltcan move smoothly over the surface, thereby preventing formation of arough sheet due to thickened melt not spread smoothly. Further, toprevent formation of rough sheet, the grooves may be formed such thatprotrusions converge in one point to collect the melt. As shown in FIG.36, for example, the oblique grooves collect the melt at the centralportion.

[0134] The above described polygonal roller may have protrusions in thebase as shown in any of FIGS. 1 to 4 and FIGS. 15 to 23. Even in thecase of FIG. 28, a sheet can be manufactured without having anyprotrusions. This is because the grooves in the surface as well as thespaces between the planar sides of the polygonal body can be controlledto provide desired shapes.

[0135] As described above, when the base with protrusions is moved(FIGS. 25 to 28 and FIG. 31) or when the crucible is moved with the basefixed (FIG. 24), the shapes of the protrusions in the base can bedesigned in consideration of the shape of the sheet to be produced,regardless of whether or not the base moves.

[0136] A high-purity coating may be applied to the surface of the basewith protrusions. Further, a high-purity coating may be applied to thesurfaces of the protrusions. Preferably, the coating material includesat least one of silicon carbide (SiC), boron nitride (BN) siliconnitride (SiNx), and pyrolitic carbon. Namely, the presence of thehigh-purity coating on the surface of the base prevents contamination ofthe sheet. When silicon is used as the melt, in particular, siliconcarbide, silicon nitride, pyrolitic carbon or the like can be used. Inother words, a coating containing at least one material on theprotrusions can prevent the melt from being poured into the grooves ofthe protrusions. A material with low wettability with respect to thesilicon melt prevents such melt from entering the grooves. Even in thecase of a material with high wettability with respect to the siliconmelt such as pyrolitic carbon, if the material is formed in layers, thesheet can be readily removed from the base even under the condition thatthe sheet adheres to the planar surface, because of the protrusions inthe surface of the base and the layered structure.

[0137] Although the surface is coated with at least one material,preferably, thermal expansion coefficients of these materials do notdiffer largely. This is because the difference in thermal expansioncoefficient between the surface of the base to be subjected to hightemperature and the base to be cooled may cause a coating to come off.

[0138] As described above, a high-quality sheet can be obtained byappropriately controlling the material or thickness of the coating onthe surfaces of the protrusions. The sheet thus formed can be cut tohave a desired size (length) with use of a microcutter, YAG laser or thelike, or directly used as it is.

[0139] Now, referring to FIG. 29 which shows a schematic diagram of aninternal portion of the sheet manufacturing apparatus with the roller,the sheet manufacturing method will be described. A crucible is denotedby 25, a crucible table with an elevator is denoted by 26, and a heaterfor temperature adjustment is denoted by 27. Crucible 25 containing themelt is disposed on crucible table 26. The roller having a coolingportion is arranged above the crucible table. In the drawing, it can beseen that linear protrusions are formed in the rotation direction in thesurface of the roller so as to be dipped into the melt in the crucible.

[0140] Referring now to FIG. 30 of a schematic diagram which shows thesheet manufacturing apparatus, the method of manufacturing the sheetwill be described. A pipe for additionally introducing material, aheat-resisting material, a chamber, and a guide roller fortransportation of sheet are respectively denoted by 28, 29, 30, and 31.Although a cylindrical roller is employed herein, a polygonal roller asshown in FIGS. 27 and 28 may be employed. Further, the surface structureof the roller may be as shown in any of FIGS. 1 to 4 and FIGS. 15 to 23.

[0141] The sheet manufacturing apparatus is provided in a tightly closedchamber 30 and gas exchange is performed with use of inert gas afterevacuation. Examples of the inert gas are argon and helium, where argonis more preferable in terms of cost.

[0142] Preferably, heat-resisting material 29 and a plurality of heatersfor temperature adjustment 27 are arranged in chamber 30 to accuratelycontrol the melt temperature. Accurate control of the ambienttemperature in chamber 30 and the melt temperature enables the sheets tobe produced stably with higher reproducibility. The temperature of themelt is, preferably, at least the melting point of the metal material orthe semiconductor material. If the temperature of the melt is near themelting point, when the cooled base is dipped in the melt, the meltsurface may be coagulated.

[0143] Although crucible 25 containing the melt is placed on crucibletable 26, crucible table 26 is preferably provided with an elevator, soas to maintain the base in the melt at the same depth for forming thesheet on the base.

[0144] For maintaining the surface of the melt at the same level,polycrystalline body of the same material as the melt is molten orpowder is sequentially added with use of pipe 28. However, the method ofmaintaining the level of the melt is not particularly limited. Note thatfluctuation of the surface of the melt should be minimized. This isbecause the fluctuation of the surface of the melt may impair uniformityof the resultant sheet.

[0145] The present invention will be further described in detail withreference to FIG. 31. The apparatus of the present invention is suitablefor use in manufacturing a silicon sheet. FIG. 31 is a cross sectionalview of a cooling roller of the present embodiment. The cooling rollerof the present embodiment has protrusions in its surface. Theprotrusions are in the shape of V or U groove, and arranged in adot-like pattern or a linear pattern. The silicon sheet is produced fromthe protrusions by solidification and crystal growth, and can be readilyand continuously pulled out. The detailed description is given in thefollowing.

[0146] Referring to FIG. 31, reference numerals 32, 33, 34, and 35respectively denote a cooling roller, a peak of the V groove of thecooling roller (a top portion of the protrusion of the cooling roller),a trough of the V groove, and gas nozzle for introducing coolant gas.Any material can be used for cooling roller 32 if it has resistance tohigh temperature. Examples of the material include molds of high-puritycarbon, fire-resisting ceramics, silicon nitride, and boron nitride. Thecarbon material is, preferably, inexpensive and has high workability. Ametal material may also used for cooling roller 32. Any material may beused without any problem, which has a melting point higher than that ofsilicon and which does not adversely affect the semiconductor propertiesof the resultant silicon sheet. Even when the metal material has amelting point lower than that of silicon, if coolant gas or water issupplied to maintain the temperature below the melting point of thatmetal, such metal material can be used without causing any particularproblem. A plurality of peaks 33 of V grooves are arranged in an annularmanner in the surface of cooling roller 32, having a prescribed pitchand depth. The cooling roller has a hollow shape for the coolant gas orwater.

[0147] Gas nozzle 35 is coaxially provided inside cooling roller 32,which is adapted to direct externally supplied coolant gas toward thepeaks 33 of V grooves. Although the type of the coolant gas is notparticularly limited since it is not brought into direct contact withthe silicon melt, preferably, the coolant gas is an inert gas such asN₂, He, or Ar. He or a mixture of He and N₂ is preferable in terms ofcooling ability, whereas N₂ is preferable in terms of cost. Aftercooling roller 1 from inside, the coolant gas is exhausted. However,preferably, the evacuated gas after cooling roller 1 may be circulatedby an externally provided heat exchanger (not shown) for furtherreduction of cost.

[0148] As described above, cooling roller 32 of the present embodimenthas in its surface V grooves as shown in FIG. 31. Greater thickness ofthe peaks and troughs of the V grooves of cooling roller 32 may provideenhanced cooling effect. Note that a greater surface area, for exampleachieved by provision of protrusions inside cooling roller 32, canfurther enhance cooling effect. Namely, it is preferable to increase thecontact area of the coolant gas by providing protrusions inside coolingroller 32.

[0149] Cooling roller 32 is dipped into the silicon melt to form asilicon sheet on its surface. As shown in FIG. 32, cooled peaks 33 of Vgrooves are dipped into the silicon melt, so that the crystal nuclei ofsilicon are first produced at the peaks 33 of the V grooves. Crystalgrowth starts from based on the crystal nuclei. As crystal growthproceeds, the silicon crystals from adjacent peaks 33 of the V groovescombine to produce a silicon sheet 60.

[0150] The resultant silicon sheet has curved portions on the melt side.Namely, the portions where the crystal nuclei are first formedcorrespond to the peaks of the V or U grooves of cooling roller 32, andthe silicon crystals which grow therefrom are sufficiently cooled in aninitial stage of crystal growth. Thus, the shape of the sheet generallyreflects the surface structure of the cooling roller. However, the sheetmay not conform to the surface structure of the cooling rollercompletely, when removability is considered. If the silicon sheetconforms completely to the surface structure of the cooling roller 32,it means that the sheet tightly adheres to cooling roller 32. Thus, asilicon sheet with sufficient removability is not obtained.

[0151] On the melt side, the silicon melt is, partially, not completelycooled and still remains in the melt state. Such silicon provides curvedportions with its force of gravity. Thus, a silicon sheet in a specificshape of the present invention is produced. In this method, aninexpensive silicon sheet can be provided because the slicing loss issaved and the curved portions provide corresponding reduction in theamount of silicon used.

[0152] Further, with use of the cooling roller 32 having V or U grooves,corrugated or wave-like silicon sheet can be directly formed on thecooling roller under manufacturing conditions of the silicon sheet.Namely, the silicon sheet has a feature on the side in contact with thecooling roller, in addition to the feature of the curved portions on themelt side.

[0153] If the coolant gas for cooling roller 32 is in sufficient or therotating speed is sufficiently high, the crystal nuclei of siliconproduced at the peaks of the V or U grooves of cooling roller 32 rapidlygrow. Then, crystal growth of silicon starts, and the direction ofcrystal growth is determined by the cooling ability of the coolant gasor the rotating speed of cooling roller 32. If the cooling ability ofthe coolant gas is sufficient, the rotating speed of the roller 32 ishigh, and the melt temperature is low, silicon crystals grow after thecrystal nuclei are produced along peaks 33 of the V or U grooves ofcooling roller 32 in the direction toward the melt side, as shown inFIG. 32. Conversely, if the cooling ability of the coolant gas isinsufficient, the rotating speed of cooling roller 32 is low, and themelt temperature is high, silicon crystals grow after the crystal nucleiare produced along peaks 33 of the V or U grooves of cooling roller 32in the direction of the grooves of the cooling roller, as shown in FIG.33.

[0154] In any of the cases, the surface structure of the cooling rollerand the silicon sheet would not completely conform to each other asshown in FIGS. 32 and 33. If the surface structure of the cooling rollercompletely conform to the silicon sheet, adhesion strength between thesurface of the cooling roller and the silicon sheet is high, wherebyremoval becomes difficult. Thus, it is not desirable for serialproduction. As described above, by appropriately determining theconditions, a silicon sheet in a desired shape can be produced.

[0155] Although a silicon sheet in a corrugated or wave-like shape canbe manufactured with use of a silicon wafer, such manufacture involvespatterning by application of an oxide film or resist and subsequentetching. The resultant silicon sheet is polycrystalline, and thereforecrystal grains of various plane directions exist. Thus, it is difficultto form the sheet in the corrugated or wave-like shape even bypatterning since the etching speed varies according to the planedirection. Namely, a silicon sheet can be obtained inexpensively in thismethod of directing forming the silicon sheet in the corrugated orwave-like shape from the silicon melt.

[0156] As described above, by changing the cooling ability of thecoolant gas, the rotating speed of the cooling roller, the melttemperature of the silicon melt and the like, a silicon sheet in adesired shape can readily be produced. In any case, the melt side hascurved portions as in the present invention.

[0157] Preferably, only cooling roller 32 rotates with gas nozzle 35shown in FIG. 31 fixed. If gas nozzle 35 rotates with cooling roller 32,the same portion is always cooled, whereby it becomes difficult touniformly form the silicon sheet on the surface of cooling roller 32.Such a structure enables the internal portion of cooling roller 32 to beuniformly cooled, so that a uniform silicon sheet can be produced.

[0158] Although gas nozzles 35 are shown in FIG. 31 being above eachother, the arrangement is not particularly limited to this. For thepurpose of uniformly cooling, a plurality of gas nozzles 35 are arrangedin radial directions rather than in one direction. Thus, the crystalnuclei can be produced more effectively on the surface of cooling roller32, so that a uniform silicon sheet can be produced.

[0159] Preferably, an SiC film is formed on the surface of the V or Ugrooves in cooling roller 32. In the case where cooling roller 32 isformed of graphite, crystal nuclei of silicon are produced on a graphitesurface, and therefore the silicon sheet would be produced on thegraphite surface. In this case, when the silicon sheet is removed fromthe base, graphite may come off with the silicon sheet. The graphiteadhering to the silicon sheet may be readily removed by etching with useof a mixture of nitric acid and hydrofluoric acid, but the number ofprocesses cost increase. If a cooling roller of graphite is used, suchroller is formed by cutting and shaping a graphite mold. It is almostimpossible to completely remove the resulting cutting chips. The chipsmay fall into the silicon melt, thereby causing a problem associatedwith contamination of the silicon melt.

[0160] To solve the aforementioned problems, it is preferred that thesurface of the cooling roller is coated with an SiC film or the like.Such a surface structure alleviates the problem of the graphite powderfalling from cooling roller 32 and contamination of the silicon sheet.Particularly, the SiC film is preferably formed of a ceramic film, forwhich a technique for forming a high-purity film is available.

[0161] In addition, high adhesion strength of the graphite coolingroller and the SiC film prevents SiC from adhering to the producedsilicon sheet when removing the sheet from the base.

[0162] Cooling roller 32 has in its surface V or U grooves, on which theSiC film is formed. As such, the SiC film at the peaks of the V or Ugrooves is dipped in the silicon melt. More specifically, the crystalnuclei of silicon are produced on the SiC film for production of thesilicon sheet. Thus, the contact area of the cooling roller 32 and thesilicon sheet becomes extremely small, whereby the adhesion strength ofthe silicon sheet and the cooling roller decreases. As a result,removability of the silicon sheet from cooling roller 32 improves.

[0163] A pitch of the V or U grooves in the surface of cooling roller 32is at least 0.05 mm and at most 5 mm. If a pitch is smaller than 0.05mm, silicon crystals begin to grow from peaks 33 of the V grooves tocombine with those from adjacent peaks 33 at troughs 34 of the Vgrooves. As a result, the crystal grain size of the silicon sheet wouldbe about half the pitch. Thus, the pitch smaller than 0.05 mm is notdesirable when the sheet is to be used for a solar cell. A greaternumber of peaks of V or U grooves increases the adhesion area of thesilicon to cooling roller 32. As a result, the adhesion strength of thesilicon sheet and the cooling roller increases, whereby removal becomesdifficult.

[0164] If a pitch of the grooves is greater than 5 mm, on the otherhand, a longer time is required for the silicon crystals growing frompeaks 33 to combine with one another to form a silicon sheet. In such acase, for example, the rotating speed of cooling roller 32 needs to bedecreased. As a result, productivity decreases and inexpensive siliconsheet cannot be provided.

[0165] Preferably, the depth of the V or U grooves of cooling roller 32is at least 0.05 and at most 5 mm. The height determines the dippingdepth of cooling roller 32. More specifically, a greater depth allowsthe greater portion of the protrusions to be dipped into the melt, andvice versa. The silicon crystals grow in the silicon melt or near thesurface of the silicon melt. Thus, the dipping depth needs to beappropriately determined as it determines the thickness of the sheet tobe produced.

[0166] As described above, the V or U groove in the surface of thecooling roller allows the crystal grain size or silicon sheet thicknessto be relatively readily controlled. The crystal grain size or siliconsheet thickness also depends on the temperature of a heater, the flowrate of coolant gas flowing through the cooling roller, the rotationspeed of the cooling roller and the like, which can be appropriatelydetermined to provide a silicon sheet with a desired thickness.

[0167] According to the present invention, a bar-like remover can bearranged in a mechanism for pulling out the produced silicon sheet.Since cooling roller 32 has V or U grooves, it is possible to arrangethe leading end of the bar-like remover along trough 34. The shape ofthe remover may be for example in a comb, bar or polygon-like shape,while not limiting. It is preferred that its shape conforms to trough 34and the height of the remover is smaller than the depth of the V or Ugrooves. Namely, any shape may be employed which conforms to trough 34of cooling roller 32 and not is not dipped into the silicon melt. Such astructure enables the silicon sheet to be easily and continuously pulledout without complicating the apparatus. There is no need to stop theoperation of the apparatus even if the silicon crystals break or fallduring crystal growth, such silicon crystals simply fall down into themelt, whereby continuous operation is enabled.

[0168] Examples of the material for the remover may include a mold bodyof carbon, ceramics, tungsten, silicon nitride, boron nitride or thelike, which are all materials stable in an inert ambient at hightemperature, while not limiting. It is preferably the same material asthe cooling roller because such material is maintained at the hightemperature as being near the silicon melt and because such materialdoes not cause damage to the surface of the cooling roller. Any shapeconforming to the shape of the V or U groove may be employed, and theshape of the remover is limited to the bar-like shape. It may also be ina fork-like shape.

[0169] As described above, provision of the V or U groove in the surfaceof cooling roller 32 and a remover conforming thereto enables theproduced silicon sheet to be readily removed from the cooling roller.The removed silicon sheet is transported by a guide roller to outsidethe chamber. Thus produced silicon sheet is cut by a microcutter, YAGlaser or the like to have a desired size (length) and to provide siliconwafers.

[0170]FIG. 34 shows a general structure of a silicon sheet manufacturingapparatus. A silicon melt, crucible, heater for temperature control,guide roll for transportation, and silicon sheet are respectivelydenoted by reference numerals 36, 37, 38, 39, and 40. The silicon sheetmanufacturing apparatus is arranged in a tightly sealed chamber, and gasexchange is performed by inert gas after evacuation. The gas may be forexample Ar or He, but Ar is more preferable in terms of cost.Preferably, a plurality of heaters are arranged in the chamber foraccurately controlling the temperature of the silicon melt. Accuratecontrol of the temperature of the silicon melt enables the production ofthe silicon sheet with sufficient reproducibility. The temperature ofthe silicon melt is preferably at least the melting point of the silicon(about 1420° C.). Near the melting point, the dipping of the cooledroller into the melt may cause solidification at the surface of themelt, and therefore the melt temperature is preferably maintained at1430° C. or higher.

[0171] Preferably, the crucible containing the melt silicon may beprovided with an elevator. This is because the cooling roller must bedipped into the silicon melt always at the same depth to produce thesilicon sheet on the cooling roller. The method of maintaining theposition of the surface of the melt at the same level may include, forexample, melting polycrystalline silicon ingot or introduction ofsilicon powders, while not limiting. However, it is preferable thatfluctuation of the surface of the silicon melt is minimized.

[0172] The embodiments of the present invention will now be described.

[0173] First Embodiment

[0174] Although one embodiment of the method of manufacturing the sheetis disclosed herein, the scope of the present invention is not limitedto this. In the present embodiment, a silicon sheet is manufactured.

[0175] A silicon material with boron concentration adjusted to haveresistivity of 2 Ω·cm was introduced into a crucible of quartz, whichwas protected by a crucible formed of high-purity carbon. The cruciblewas fixed in a chamber as shown in FIG. 30. The chamber was evacuated tohave a pressure at or below 2×10⁻⁵ torr. Thereafter, Ar gas wasintroduced to the chamber and the pressure was recovered to anatmospheric pressure. Subsequently, a flow of Ar gas was maintained fromabove the chamber, with a flow rate of 2 L/min.

[0176] Then, the temperature of the heater for melting silicon was setat 1500° C. and silicon was completely brought into a molten state. Themelt surface was lowered as the silicon material melts. Thus, a siliconmaterial was added by a pipe for introducing additional silicon materialto maintain the position of the surface at a prescribed level. Then, thetemperature of the silicon melt was set at 1430° C., which temperaturewas maintained for 30 minutes for stabilization. Then, without rotatingthe roller, nitrogen gas was sprayed to the internal portion of theroller with a flow rate of 700 L/min for cooling.

[0177] In this case, a base having linear and planar protrusions asshown in FIG. 3 was used. A pitch of linear protrusions was 1 mm and awidth of planar protrusions was 1 mm. The depth of grooves was 1 mm andthe outer dimension of the base was 50 mm×50 mm. In this case, apolygonal roller as shown in FIG. 27 was employed.

[0178] Thereafter, the quartz crucible was gradually elevated until itreaches the position allowing the base to be dipped into the melt by 3mm. As such, the roller was rotated at 0.5 rpm for producing a siliconsheet. Immediately before the roller completes one rotation, thecrucible table was lowered and dipping was stopped. The silicon sheetwas pulled out when the chamber temperature was lowered to the roomtemperature. Then, the produced silicon sheet could be readily removedfrom the base. The silicon sheet had substantially the same size as thebase, i.e., 50 mm×50 mm. The produced silicon sheet was substantially inthe shape as shown in FIG. 11, where the maximum thickness was about 0.4mm and the minimum thickness was about 0.25 mm.

[0179] Then, with use of the produced silicon sheet, a solar cell wasmanufactured. The silicon sheet was etched by a solution of nitric acidand hydrofluoric acid and cleaned. Then, alkali etching was performed bysodium hydroxide. Then, an n layer was formed on a p type substrate bydiffusion of POCl₃. After removing the PSG film formed on the surface ofthe sheet by hydrofluoric acid, a silicon nitride film was formed on then layer which was to be front side of the solar cell by plasma CVD.Then, the n layer formed on the back side of the solar cell was etchedand removed by a solution of nitric acid and hydrofluoric acid to exposethe p substrate. The back electrode and a p+ layer are formed thereon atthe same time. Then, an electrode on the light receiving side was formedby screen printing. At the time, screen printing was performed such thatthe electrode was formed on the planar portion of the silicon sheet asshown in FIG. 14. Thereafter, solder dipping was performed to complete asolar cell. Thus manufactured solar cell was measured for its propertiesunder irradiation condition of AM1.5, 100 mW/cm². The results wereshort-circuit current of 30.3 (mA/cm²), open-circuit voltage of 590(mV), fill factor of 0.69, and efficiency of 12.3(%).

[0180] Second Embodiment

[0181] A silicon sheet and solar cell were manufactured in accordancewith the same method as in the first embodiment except that the surfacestructure of the polygonal roller included only linear protrusions asshown in FIG. 1. A pitch of the linear protrusions of the base was 1 mmand the depth of the grooves was 1 mm. The surface of the sheet did nothave any planar portion, so that a sub electrode was formed in parallelwith the groove as shown in FIG. 13. The silicon sheet was in the sameshape as in FIG. 5, where the thickness of the sheet was about 0.5 mmand the manufactured solar cell showed the following measurementresults: short-circuit current of 28.3 (mA/cm²), open-circuit voltage of580 (mV), fill factor of 0.61, and efficiency of 10.0(%).

[0182] Third Embodiment

[0183] A silicon sheet and a solar cell were manufactured in the samemethod as in Embodiment 1 except that the surface structure of thepolygonal roller included only dot-like protrusions as shown in FIG. 2.A pitch of the dot-like protrusions of the base was 0.1 mm and the depthof the grooves was 0.1 mm. The produced sheet did not have any planarportion, so that an electrode was formed in parallel with the sides ofthe base. The produced sheet was in the same shape as in FIG. 8, havinga thickness of about 0.5 mm. The manufactured solar cell showed thefollowing measurement results: short-circuit current of 29.8 (mA/cm²),open-circuit voltage of 587 (mV), fill factor of 0.67, and efficiency of11.7(%).

[0184] Fourth Embodiment

[0185] A silicon sheet and solar cell were manufactured in the samemethod as in the first embodiment except that the surface structure ofthe polygonal roller had only dot-like protrusions and planarprotrusions as shown in FIG. 4. A pitch of the dot-like protrusions ofthe base was 1.5 mm and the width of the planar protrusions was 1 mm.The depth of the grooves was 0.5 mm. The produced sheet was in the shapeas shown in FIG. 12, having the thickness of about 0.3 mm. Themanufactured solar cell showed the following measurement results:short-circuit current of 30.4 (mA/cm²), open-circuit voltage of 588(mV), fill factor of 0.69, and efficiency of 12.3(%).

[0186] Fifth Embodiment

[0187] A silicon sheet and a solar cell were manufactured in the samemethod as in the first embodiment except that the flow rate of thecoolant gas in the polygonal roller was 1200 L/min. The produced siliconsheet was in the shape shown in FIG. 11, having a thickness of about 0.5mm. The manufactured solar cell showed the following measurementresults: short-circuit current of 29.9 (mA/cm²), open-circuit voltage of588 (mV), fill factor of 0.68, and efficiency of 12.0(%).

[0188] Sixth Embodiment

[0189] A silicon sheet and a solar cell were manufactured in the samemethod as in the first embodiment except that the flow rate of thecoolant gas in the polygonal roller was 500 L/min. The produced siliconsheet was in the shape as shown in FIG. 11, having a thickness of about0.7 mm. The manufactured solar cell showed the following measurementresults: short-circuit current of 30.1 (mA/cm²), open-circuit voltage of590 (mV), fill factor of 0.68, and efficiency of 12.1(%).

[0190] Comparison 1

[0191] A silicon sheet was manufactured in the same method as in thefirst embodiment except that the flow rate of the coolant gas in thepolygonal roller was 50 L/min. The produced silicon sheet was in a shapesimilar to that shown in FIG. 11, but the silicon sheet could not beremoved from the polygonal roller. Thus, a solar cell could not bemanufactured.

[0192] Comparison 2

[0193] A silicon sheet was manufactured in the same method as in thefirst embodiment, and a solar cell was manufactured in the same methodas in the first embodiment except that an electrode was formed in theportion other than the planar portion. The produced sheet has athickness of about 0.5 mm, and the manufactured solar cell showed thatfollowing measurement results: short-circuit current of 29.1 (mA/cm²),open-circuit voltage of 585 (mV), fill factor of 0.64, and efficiency of10.9(%).

[0194] Comparison 3

[0195] A silicon sheet was manufactured in the same method as in thefirst embodiment except that the surface structure of the polygonalroller had only a planar portion, free of protrusions, and a solar cellwas also manufactured. The silicon sheet was mainly formed by dendriticgrowth and the surface structure of the produced silicon sheet wasuneven. The manufactured solar cell showed the following measurementresults: short-circuit current of 28.3 (mA/cm²), open-circuit voltage of580 (mV), fill factor of 0.65, and efficiency of 10.7(%).

[0196] Seventh Embodiment

[0197] Although one embodiment of a method of manufacturing a sheet isdescribed here, the scope of the present invention is not limited tothis. In the present embodiment, a silicon sheet was manufactured. Asilicon material of which boron concentration has been adjusted to haveresistivity of 0.5 Ω·cm was introduced to a crucible of quartz protectedby a high-purity carbon crucible. The crucible is then fixed in achamber as shown in FIG. 30.

[0198] The chamber was evacuated to have a pressure at or below 2×10⁻⁵torr. Then, Ar gas was introduced into the chamber and the pressure wasrecovered to an atmospheric pressure. Subsequently, Ar gas was allowedto flow from above the chamber at 2 L/min. Then, the temperature of aheater for melting silicon was set at 1500° C. to completely bring thesilicon into a molten state. At the time, the silicon material melts andhence decreases its surface level. Thus, the silicon material is furtherintroduced by a pipe to adjust the surface level to a prescribedposition. Then, the temperature of the silicon melt is set at 1450° C.and maintained thereat for 30 minutes for stabilization. Then, nitrogengas is sprayed to the internal portion of the roller at 1000 L/minwithout rotating the roller for cooling.

[0199] In this case, a cylindrical roller having a surface structureincluding linear and planar protrusions as shown in FIG. 4 was used. Apitch of dot-like protrusions was 2 mm, a width of the planarprotrusions was 1 mm, and the roller had a diameter of 800 mm.

[0200] Then, the quartz crucible was gradually elevated up to a levelwhere the base was dipped into the melt by 4 mm, where the roller wasrotated at 5 rpm for production of the silicon sheet. Immediately beforethe roller completes one rotation, the crucible table was lowered tostop heating. The silicon sheet was pulled out when the chamber attainsto a room temperature. In this case, the silicon sheet could readily beremoved from the base. The silicon sheet was cut to have a size of 50mm×40 mm. The silicon sheet was in the shape as shown in FIGS. 12, wherethe maximum thickness was about 0.3 mm and the minimum thickness wasabout 0.2 mm.

[0201] Then, a solar cell was manufactured using the produced siliconsheet. The produced silicon sheet was etched and cleaned using asolution of nitric acid and hydrofluoric acid and then subjected toalkali etching using sodium hydroxide. Thereafter, an n layer formed ona p type substrate by diffusion of POCl₃. A PSG film formed on thesurface of the sheet was removed by hydrofluoric acid, and a siliconnitride film was formed on the n layer to be a light receiving side ofthe solar cell by plasma CVD. Then, the n layer formed on the back sideof the solar cell was etched and removed by a solution of nitric acidand hydrofluoric acid, and the back electrode and p+ layer were formedat the same time. Then, an electrode on the light receiving side wasformed by screen printing. Thereafter, solder dipping is performed toproduce a solar cell. The manufactured solar cell was measured for itscell properties under radiation conditions of AM1.5, 100 mW/cm². Themeasurement results showed short-circuit current of 30.5 (mA/cm²),open-circuit voltage of 589 (mV), fill factor of 0.70, and efficiency of12.6(%).

[0202] Comparison 4

[0203] A silicon sheet was manufactured by the same method as in theseventh embodiment except that the rotating speed of a cylindricalroller was 15 rpm. The produced silicon sheet had holes, becausecrystals were not formed at the grooves of the sheet.

[0204] Eighth Embodiment

[0205] A silicon sheet and a solar cell were manufactured in the samemethod as in the seventh embodiment except that the surface structure ofthe cylindrical roller had only linear and planar protrusions as shownin FIG. 18. A pitch of the linear protrusions of the base was 2 mm, adepth of the grooves was 0.1 mm, and width of the planar protrusion was10 mm. The produced silicon sheet was in the shape as shown in FIG. 18,having a thickness of about 0.4 mm. The manufactured solar cell showedthe following measurement results: short-circuit current of 29.8(mA/cm²), open-circuit voltage of 588 (mV), fill factor of 0.68, andefficiency of 11.9(%).

[0206] Ninth Embodiment

[0207] A silicon sheet and a solar cell were manufactured in the samemethod as in the seventh embodiment except that the surface structure ofthe cylindrical roller had only dot-like protrusions and planarprotrusions as shown in FIG. 19. A pitch of the linear protrusions ofthe base was 2 mm, a depth of the grooves was 1 mm, and a width of theplanar protrusions was 10 mm. The produced silicon sheet was in theshape as shown in FIG. 19, having a thickness of about 0.4 mm, themanufactured solar cell showed the following measurement results:short-circuit current of 29.9 (mA/cm²), open-circuit voltage of 589(mV), fill factor of 0.67, and efficiency of 11.8(%).

[0208] Tenth Embodiment

[0209] Although one embodiment of a method of manufacturing a sheet isdescribed herein, the scope of the present invention is not limited tothis. In the present embodiment, a silicon sheet was manufactured.

[0210] A silicon material of which boron concentration has been adjustedto have resistivity of 1 Ω·cm was introduced into a crucible of quartzprotected by a high-impurity carbon crucible and brought into a moltenstate. Then, as shown in FIG. 24, a crucible having at its bottom anopening is placed on the base with protrusions. In this case, the basewith linear and planar protrusions as shown in FIG. 3 was used. A pitchof the linear protrusions was 2 mm, a width of the planar protrusionswas 2 mm, and a depth of the grooves was 0.1 mm. At the time, the basewith protrusions was cooled down with a flow rate of coolant gas of 500L/min. Thereafter, molten silicon is introduced into the crucible and,at the same time, the crucible is moved at a speed of lm/min to producea silicon sheet. The produced silicon sheet was substantially the sameas the base in size, having 50 mm×50 mm. The produced silicon sheet hadthe maximum thickness of about 0.3 mm and the minimum thickness of about0.15 mm.

[0211] Then, a solar cell was manufactured with use of the producedsilicon sheet. The produced silicon sheet was subjected to alkalietching using sodium hydroxide. Then, an n layer was formed on a p typesubstrate by diffusion of POCl₃. The PSG film formed on the surface ofthe sheet was removed by hydrofluoric acid, and a TiO₂ film was formedon the n layer on the front side of the solar cell. Then, the n layerformed on the back side of the solar cell was etched and removed by asolution of nitric acid and hydrofluoric acid to expose the p substrate,on which the back electrode and p+ layer were simultaneously formed.Then, an electrode on the front side was formed by screen printing. Thescreen printing was performed to form an electrode at the planar portionof the silicon sheet. Then, solder dipping was performed to produce asolar cell. The produced solar cell was measured for its cell propertiesunder irradiation conditions of AM1.5, 100 mW/cm². The measurementresults showed short-circuit current of 25 (mA/cm²), open-circuitvoltage of 571 (mV), fill factor of 0.65, and efficiency of 9.3(%)

[0212] Eleventh Embodiment

[0213] A sheet was manufactured in the same method as in the tenthembodiment except for usage of nickel. The produced nickel sheet was inthe shape as shown in FIG. 11, having a thickness of about 0.4 mm.

[0214] Twelfth Embodiment

[0215] A sheet was manufactured in the same method as in the tenthembodiment except for usage of aluminum and the base of FIG. 2. A pitchof dot-like protrusions was 1 mm, and a depth of the grooves was 0.6 mm.The produced aluminum sheet was in the shape as shown in FIG. 8, havinga thickness of about 0.3 mm.

[0216] Thirteenth Embodiment

[0217] Although one embodiment of a method of manufacturing a siliconsheet and solar cell properties is described herein, the scope of thepresent invention is not limited to this. In the present embodiment, thebase of the cooling roller had V grooves, for the purpose of observingvariation in the shape of the silicon sheet according to the rotatingspeed.

[0218] High-purity silicon (purity of 99.999999999%) and, as a dopantthereof, a master alloy with boron, were prepared. A silicon material ofwhich boron concentration had been adjusted to have resistivity of 2Ω·cm was introduced into a quartz crucible protected by a high-puritycarbon crucible. The crucible was fixed in a chamber (chamber wall isnot shown) of the apparatus shown in FIG. 34. The chamber was evacuatedto attain a pressure at or below 2×10⁻⁵ torr. Then, Ar gas wasintroduced into the chamber so that the chamber attained an atmosphericpressure. Subsequently, a flow of Ar gas was maintained from above intothe chamber at 2 L/min. Then, the temperature of a heater for meltingsilicon is set at 1500° C. to bring silicon completely in a moltenstate. At the time, as silicon material melted, its surface levelsignificantly decreased. Thus, additional silicon material is introducedto adjust the surface of the melt at a prescribed level. Then, thesilicon melt temperature is set at 1430° C., which temperature ismaintained for 30 minutes for stabilization. Nitrogen gas is sprayed tothe internal portion of the cooling roller at a flow rate of 700 L/minwithout rotating the cooling roller having a SiC film of 10 μm. A pitchof the V grooves of the cooling roller then used was 1 mm, and a depthof the grooves was 1 mm. Thereafter, the quartz crucible was graduallyelevated to a level where the peaks of the V grooves of the roller weredipped into the melt by 3 mm. At this point, the roller was rotated at arotating speed of 0.5 rpm, 5 rpm, and 10 rpm to produce a silicon sheet.The resultant silicon sheet had a thickness and average crystal grainsize as shown in the following Table 1. TABLE 1 ROTATING SPEED THICKNESSGRAIN SIZE (rpm) (μm) (mm) 0.5 1000  0.8 5 600 0.7 10 300 0.5

[0219] As the rotating speed increases, the thickness of the siliconsheet as well as average crystal grain size decrease. Namely, bycontrolling rotation of the cooling roller, the thickness of the siliconsheet formed on the cooling roller can readily be controlled. Inaddition, the shape of the produced silicon sheet had a cross section asshown in FIG. 33 at 0.5 rpm, whereas the silicon sheet had a crosssection as shown in FIG. 32 at 5 rpm, and 10 rpm.

[0220] Then, a solar cell was manufactured in accordance with theprocedure shown in conjunction with FIG. 35 with use of the producedsilicon sheet. The silicon sheet was etched and cleaned by a solution ofnitric acid and hydrofluoric acid and then subjected to alkali etchingusing sodium hydroxide. Then, an n layer was formed in a p typesubstrate by diffusion of POCl₃. After a PSG film formed on the surfaceof the base was removed by hydrofluoric acid, a titanium oxide film wasformed on the n layer on the front side of the solar cell as aanti-reflection film. Then, the n layer formed on the back side of thesolar cell was etched and removed by a solution of nitric acid andhydrofluoric acid to expose the p substrate, on which the back electrodeand p+ layer were formed at the same time. Thereafter, the electrode onthe front side was formed by screen printing and solder dipping isperformed to produce a solar cell. The manufactured solar cell wasmeasured for its cell properties under illumination conditions of AM1.5,100 mW/cm². The cell properties of the prototype solar cell are as shownin the following Table 2. TABLE 2 ROTATING SPEED Jsc Voc FILL EFFICIENCY(rpm) (mA/cm²) (mV) FACTOR (%) 0.5 24.8 532 0.659 8.7 5 23.9 510 0.6688.1 10 22.5 511 0.645 7.4

[0221] Although there is some difference in the properties of the solarcells according to the number of rotation, inexpensive solar cells canbe provided because solar cell properties are given despite the simplecell process.

[0222] Fourteenth Embodiment

[0223] Although one embodiment of a method of manufacturing a siliconsheet and solar cell properties are described herein, the scope of thepresent invention is not limited to this. In the present embodiment, acooling roller with U grooves is used, for the purpose of observingvariation in the shape of the silicon sheet according to the flow rateof the coolant gas.

[0224] A master alloy with high-purity silicon (purity of 99.999999999%)and, as a dopant thereof, boron were prepared. A silicon material ofwhich boron concentration had been adjusted to have resistivity of 3Ω·cm was introduced into a quartz crucible protected by a high-puritycarbon crucible. The crucible is fixed in a chamber (chamber wall is notshown) of an apparatus shown in FIG. 34. The chamber is evacuated toattain a pressure at or below 2×10⁻⁵ torr. Then, Ar gas was introducedinto the chamber to have the atmospheric pressure. Subsequently, a flowof Ar gas is maintained from above the chamber at 5 L/min. Then, thetemperature of a heater for melting silicon is a set at 1500° C. tobring silicon completely into a molten state. Then, as the siliconmaterial melted, its surface was significantly lowered. Thus, additionalsilicon material was introduced to adjust the melt surface to aprescribed level. Then, the silicon melt temperature was set at 1450°C., which temperature was maintained for 30 minutes for stabilization.Then, nitrogen gas was sprayed to the internal portion of the coolingroller having an SiC film of 100 μm with a flow rate of 1000 L/minwithout rotating the cooling roller. A pitch of U grooves of the coolingroller then employed was 5 mm, and a depth of the U grooves was 3 mm.Thereafter, the quartz crucible was gradually elevated to a level wherethe peaks of the U grooves of the roller were dipped by 3 mm. At thatpoint, the roller is rotated at a rotating speed of 0.5 rpm forproducing the silicon sheet. Further, by performing the similaroperation, a silicon sheet was manufactured with the nitrogen gas flowrate of 800 L/min, and 500 L/min. The thickness and average crystalgrain size of the produced silicon sheet are shown in the followingTable 3. TABLE 3 FLOW RATE THICKNESS GRAIN SIZE (L/min) (μm) (mm) 1000 900 2.5 800 800 2.7 500 750 2.8

[0225] From the above table, as the flow rate of the coolant gasdecreases, the thickness of the silicon sheet also decreases. Namely, bycontrolling the flow rate of the coolant gas, the thickness of thesilicon sheet formed on the cooling roller can readily be controlled.The silicon sheet had a cross section as shown in FIG. 32 with the flowrate of 1000 L/min and 800 L/min whereas the silicon sheet had a crosssection as shown in FIG. 33 with the flow rate of 500 L/min.

[0226] Then, a solar cell was manufactured in accordance with the stepsshown in conjunction with FIG. 35 with use of the produced siliconsheet. The produced silicon sheet was etched and cleaned by a solutionof nitric acid and hydrofluoric acid and then subjected to alkalietching by sodium hydroxide. Then, an n layer was formed on a p typesubstrate by diffusion of PSG. The PSG film formed on the surface of thebase was removed by hydrofluoric acid, and a silicon nitride film wasformed on the n layer on the light receiving side of the solar cell asan anti-reflection film. Then, the n layer formed on the back side ofthe solar cell was etched and removed by a solution of nitric acid andhydrofluoric acid to expose a p substrate, and the back electrode andthe p+ layer were formed thereon at the same time. Then, an electrode onthe light receiving side was formed by screen printing. Thereafter,solder dipping was performed to produce a solar cell. The manufacturedsolar cell was measured for its cell properties under illuminationconditions of AM1.5, 100 mW/cm². The cell properties of the prototypesolar cell are shown in the following Table 4. TABLE 4 FLOW RATE Jsc VocFILL EFFICIENCY (L/min) (mA/cm²) (mV) FACTOR (%) 1000  23.3 550 0.71 9.1800 29.0 545 0.70 8.9 500 23.3 553 0.69 8.9

[0227] Although there is some difference in properties of the solarcells according to the flow rate of the coolant gas, inexpensive solarcells can be provided because solar cell properties are provided despitethe simple cell process.

[0228] Fifteenth Embodiment

[0229] Although one embodiment of a method of manufacturing a siliconsheet and solar cell properties are described herein, the scope of thepresent invention is not limited to this. In the present embodiment, acooling roller with V grooves is used, for the purpose of observingvariation in the shape of the silicon sheet according to a pitch of theV grooves.

[0230] A high purity silicon (purity of 99.999999999%) and master alloyincluded boron as a dopant were prepared. A silicon material of whichboron concentration had been adjusted to have resistivity of 2 Ω·cm wasintroduced into a quartz crucible protected by a high-purity carboncrucible. The crucible was fixed in a chamber (chamber wall is notshown) in the apparatus shown in FIG. 34. The chamber was evacuated toattain a pressure at or below 2×10⁻⁵ torr. Then, Ar gas was introducedinto the chamber to attain a room pressure. Subsequently, a flow of Argas is maintained from above the chamber at 3 L/min. Then, thetemperature of a heater for melting silicon was set at 1500° C. tocompletely bring silicon into a molten state. Then, as the siliconmaterial melted, its surface level decreased. Thus, additional siliconmaterial was introduced to adjust the surface to a prescribed level.Thereafter, the silicon melt temperature is set at 1435° C., whichtemperature was maintained for 30 minutes for stabilization. Then,nitrogen gas was sprayed to the internal portion of the cooling rollerhaving an SiC film of 200 μm with a flow rate of 500 L/min for coolingwithout rotating the cooling roller. A pitch of the V grooves of thecooling roller then used was 0.05 mm and a depth of the grooves was 1mm. Thereafter, the quartz crucible was gradually elevated to a levelwhere peaks of the V grooves of the roller were dipped by 3 mm. At thatpoint, the roller was rotated at the rotating speed of 10 rpm to producea silicon sheet. By performing a similar operation, the silicon sheetwas manufactured with a pitch of the cooling roller being 2 mm or 5 mm.The thickness and average grain size of the produced silicon sheet areshown in the following Table 5. TABLE 5 PITCH THICKNESS GRAIN SIZE (mm)(μm) (mm) 0.05 300 0.03 2 400 0.95

[0231] As a pitch of the cooling roller increases, average grain sizealso increases. Namely, by controlling the pitch of the surface of thecooling roller, the crystal grain size of the silicon sheet formed onthe cooling roller can readily be controlled.

[0232] Then, using the produced silicon sheet, a produced solar cell wasmanufactured in accordance with the steps shown in conjunction with FIG.35. The produced silicon sheet was etched by acid and the base wascleaned. Then, an n layer was formed on a p type substrate by diffusionof POCl₃. A PSG film formed on the surface of the base was removed byflouric acid, and then a silicon nitride film was formed on an n layeron the light receiving side of the solar cell as an anti-reflectionfilm. Then, the n layer formed on the back side of the solar cell wasetched and removed by a solution of nitric acid and hydrofluoric acid toexpose the p substrate, on which a back electrode and a p+ layer wereformed at the same time. After an electrode on the light receiving sidewas formed by screen printing, solder dipping was performed to produce asolar cell. The manufactured solar cell was measured for its cellproperties under illumination conditions of AM1.5, 100 mW/cm². The cellproperties of the prototype solar cell were shown in the following Table6. TABLE 6 PITCH Jsc Voc FILL EFFICIENCY (mm) (mA/cm²) (mV) FACTOR (%)0.05 22.1 530 0.71 8.3 2 21.3 538 0.69 7.9 5 21.0 542 0.71 8.1

[0233] In particular, inexpensive silicon sheet can be provided becausesolar cell properties can still be given without performing alkalietching. Although there are some differences in property due todifference in pitch of the V grooves of the roller, inexpensive solarcells can be provided because solar cell properties can be still givendespite the simple cell process.

[0234] Sixteenth Embodiment

[0235] A silicon sheet was manufactured in the same method as in thefirst embodiment except that the surfaces of the protrusions of apolygonal roller had two layers of BN and SiC films. The produced sheetexhibited good removability. The sheet was approximately in the shape asshown in FIG. 14.

[0236] Comparison 5

[0237] A silicon sheet was manufactured in the same method as in thefirst embodiment except that the polygonal roller was planar, free ofprotrusions, and having on its surface two layers of BN and SiC films.The produced sheet was partially removable.

[0238] As is apparent from the above, crystal nuclei can be firstproduced at the peaks of protrusions of the base, so that crystals fromadjacent peaks combine together to form a sheet. Therefore, aninexpensive sheet with a desired thickness and shape can be rapidlyproduced with sufficient controllability. Further, as is also apparentfrom the above, silicon nuclei can be first produced at the peaks of theprotrusions of the cooling roller, and crystals from the adjacent peakscombine above troughs of the protrusions to form a silicon sheet.Therefore, a silicon sheet with a desired thickness and crystal grainsize can be inexpensively and rapidly formed with high controllability.If an SiC film is formed on the surface of the cooling roller,contamination of the silicon sheet by the material of the cooling rollercan be prevented, whereby high quality silicon sheet can be provided.

[0239] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

1. A sheet manufacturing method, comprising the steps of: cooling a basehaving protrusions; dipping surfaces of said protrusions of said cooledbase into a melt material containing at least one of a metal materialand a semiconductor material; and forming crystals of said material onthe surfaces of said protrusions.
 2. A sheet manufacturing method,comprising the steps of: rotating a roller with protrusions on itsperipheral surface and a cooling portion capable of cooling saidprotrusions; dipping surfaces of said cooled protrusions into a meltmaterial containing at least one of a metal material and a semiconductormaterial; and forming crystals of said material on the surfaces of saidprotrusions.
 3. The sheet manufacturing method according to claim 1,wherein said protrusions have at least one of dot-like protrusions andlinear protrusions.
 4. The sheet manufacturing method according to claim2, wherein said protrusions have at least one of dot-like protrusionsand linear protrusions.
 5. The sheet manufacturing method according toclaim 1, wherein said protrusions have at least one of dot-likeprotrusions and linear protrusions in addition to planar protrusions. 6.The sheet manufacturing method according to claim 2, wherein saidprotrusions have at least one of dot-like protrusions and linearprotrusions in addition to planar protrusions.
 7. The sheetmanufacturing method according to claim 1, wherein said protrusions arecoated with a coating material of at least one of silicon carbide, boronnitride, silicon nitride and pyrolitic carbon.
 8. The sheetmanufacturing method according to claim 2, wherein said protrusions arecoated with a coating material of at least one of silicon carbide, boronnitride, silicon nitride and pyrolitic carbon.
 9. The sheetmanufacturing method according to claim 1, wherein crystal growth ofsaid material starts from said protrusions.
 10. The sheet manufacturingmethod according to claim 2, wherein crystal growth of said materialstarts from said protrusions.
 11. A sheet produced by cooling a basehaving protrusions, dipping surfaces of said protrusions of said cooledbase into a melt material containing at least one of a metal materialand a semiconductor material, wherein said sheet has a curve portionobtained by forming crystals of said material from protrusions on thesurface of said base in a curved shape.
 12. A sheet produced by coolinga base having at least one of dot-like protrusions and linearprotrusions in addition to planar protrusions and dipping surfaces ofsaid protrusions of said cooled base into a material containing at leastone of a metal material and a semiconductor material, wherein said sheethas curved portions and planar portions obtained respectively by formingcrystals of said material from said dot-like protrusions or linearprotrusions on the surface of said base in a curved shape and by formingcrystals of said material from planar protrusions on the surface of saidsubstrate in a planar shape.
 13. A sheet manufacturing apparatus,comprising: a roller having on its peripheral surface protrusions and acooling portion for cooling said protrusions; and a crucible including amelt material containing at least one of a metal material and asemiconductor material and capable of dipping said protrusions into saidmelt by rotation of said roller.
 14. A solar cell produced by forming anelectrode on a sheet formed by cooling a base with protrusions anddipping the surfaces of said protrusions of said cooled base into a meltmaterial containing at least one of a metal material and a semiconductormaterial, wherein said sheet has curved portions obtained by formingcrystals of said material from said protrusions on the surface of saidbase in a curved shape.
 15. A solar cell produced by forming anelectrode on a sheet formed by cooling a base with at least one ofdot-like protrusions and linear protrusions in addition to planarprotrusions and dipping a surface of said cooled base into a meltmaterial containing at least one of a metal material and a semiconductormaterial, wherein said sheet has curved portions and planar portionsobtained respectively by forming crystals of said material from saiddot-like protrusions or linear protrusions on the surface of said basein a curved shape and by forming said crystals of said material fromsaid planar protrusions on the surface of said base in a planar shape.16. The silicon sheet manufacturing apparatus of manufacturing a siliconsheet by solidifying a silicon melt by rotation of a cooling roller forcrystal growth, characterized in that said cooling roller has in itssurface protrusions arranged in a dot-like pattern or in a linearpattern when viewed from above.
 17. The silicon sheet manufacturingapparatus according to claim 16, characterized in that a space betweensaid protrusions is in a V or U like shape.
 18. The silicon sheetmanufacturing apparatus according to claim 16, characterized in that thesurface of said cooling roller is covered with an SiC film.
 19. Thesilicon sheet manufacturing apparatus according to claim 17,characterized in that a pitch of said V or U grooves is at least 0.05 mmand at most 5 mm.
 20. The silicon sheet manufacturing apparatusaccording to claim 16, wherein a height of said protrusions is at least0.05 mm and at most 5 mm.
 21. A silicon sheet manufacturing method ofmanufacturing a silicon sheet by solidifying a silicon melt by rotationof a cooling roller for crystal growth, characterized in that saidcrystals grow from protrusions of said cooling roller arranged in adot-like pattern or in a linear pattern when viewed from above.
 22. Asolar cell produced by rotating a roller having on its peripheralsurface protrusions having at least one of dot-like protrusions andlinear protrusions and a cooling portion for cooling said protrusions,and dipping surfaces of said cooled protrusions into a silicon melt sothat silicon crystals grow on the surfaces of said protrusions.